In this proposal, the investigators focus on maternal lipid insulin sensitivity, defined as the ability of insulin to suppress whole body lipolysis as estimated from changes in free fatty acid (FFA) concentration and glycerol turnover, in order to evaluate possible mechanisms for changes in maternal body composition and fetal growth observed during the last funding period. The specific aims are to: 1) evaluate the longitudinal changes in maternal lipid insulin sensitivity prior to and during pregnancy in lean and obese women with normal glucose tolerance (CTL) and gestational diabetes (GDM) and 2) determine if alterations in insulin signaling pathways in adipose tissue are the mechanism(s) underlying changes in maternal lipid insulin sensitivity during pregnancy.